Some display devices use electro-optical elements whose brightness changes with change in voltage applied thereto or current flowing therethrough. For example, a liquid crystal display element is a typical example of an electro-optical element whose brightness changes with change in voltage applied thereto. An organic electro-luminescence element is a typical example of an electro-optical element whose brightness changes with change in current flowing therethrough. An organic electro-luminescence element is commonly called an OLED (Organic Light Emitting Diode). A liquid crystal display element is an optical modulation element (i.e., non-self-luminous) adapted to modulate light from the light source. An OLED differs from a liquid crystal display element in that the former is a self-luminous element which can emit light by itself.
An OLED includes a plurality of organic thin films stacked one on top of another between upper and lower electrodes. These organic films serve as organic hole transporting layer and organic light-emitting layer. These films are generally thin although they vary in thickness according to the wavelength of emitted light and also for reasons including providing light enhancement effect. These films are difficult to form because they are made of organic materials. An OLED is an electro-optical element that takes advantage of light emission as a result of application of an electric field to the organic thin films. A color gray level is obtained by controlling the current value flowing through the OLED. In a display device using OLEDs as electro-optical elements, therefore, a pixel circuit is provided for each pixel that includes a drive transistor adapted to control the amount of current flowing through the OLED.
A variety of pixel circuits have been proposed. Chief among the known circuit types are four-transistor (4T) one-capacitor (1C), 4T-2C, 5T-1C and 3T-1C circuits.
All these circuits prevent image degradation caused by characteristic variation of the transistors formed by TFTs (Thin Film Transistors) and are intended to maintain a drive current constant in the pixel circuit, thus providing improved uniformity in brightness over the entire screen. In particular, when the OLED is connected to power in the pixel circuit, the characteristic variation of the drive transistor adapted to control the amount of current according to a data potential of the input video signal directly affects the emission brightness of the OLED. Therefore, the characteristic of the drive transistor, i.e., the threshold voltage, must be corrected.
Further, assuming that the threshold voltage is corrected, correcting the driving capability component (generally referred to as mobility) obtained by subtracting the components causing variation in threshold from the current driving capability of the drive transistor provides even higher uniformity.
The correction of the threshold voltage and mobility of the drive transistor is described in detail, for example, in Patent Document 1.
Patent Document 1: Japanese Patent Laid-Open No. 2006-215213